1. Field of the Invention
The present invention relates to image forming apparatuses, and more particularly, to a method for activating a oscillation mirror that oscillates in a rocking manner in an image forming apparatus having an optical scanner that scans a light beam using the oscillating mirror.
2. Description of the Related Art
Rotary optical deflectors including rotatable polygonal mirrors and resonant optical deflectors including resonantly oscillating mirrors are known as optical deflectors for use in image forming apparatuses, such as laser beam printers and digital copying machines.
Rotary optical deflectors are advantageous in that an image bearing member can be reliably scanned with a laser beam at a constant speed and activation control is easy. For this reason, the rotary optical deflectors are commonly used.
On the other hand, various types of resonant optical deflectors including resonantly oscillating mirrors have also been proposed. Compared to rotary optical deflectors including optical scanning systems using rotatable polygonal mirrors, resonant optical deflectors have the following characteristics. That is, the size of the optical deflector can be greatly reduced, power consumption is small, and surface tilting of the mirror theoretically does not occur. In addition, if the optical deflector is made of single crystal silicon (Si) manufactured by a semiconductor process, in theory, no metal fatigue occurs and high durability is obtained. Due to these characteristics, resonant optical deflectors have recently been attracting attention as elements satisfying requirements for size and cost reduction in printers.
However, in resonant optical deflectors, a deflection angle (displacement angle) of a mirror basically varies in a sine curve, and therefore the angular speed is not constant. The deflection angle of the mirror and a scanning angle of scanning light deflected and scanned by the mirror are in a constant relationship, and can be considered equivalent to each other. Therefore, in the following description, the term “deflection angle (displacement angle)” and the term “scanning angle” have similar meanings. A method for compensating for the non-constant angular speed is suggested in, for example, U.S. Pat. No. 4,859,846.
In this method, a resonant optical deflector having oscillation modes with a fundamental frequency and a frequency three times as high as the fundamental frequency is used to allow driving with a substantially triangular wave. FIG. 20 illustrates a micromirror that can be driven with a substantially triangular wave. A resonant optical deflector 12 includes rocking members 14 and 16, torsion springs 18 and 20, a drive unit 23, a drive circuit 50, detectors 15 and 32, and a control circuit 30. The micromirror has a fundamental resonance frequency and a resonance frequency about three times as high as the fundamental resonance frequency, and is driven by a resultant signal having frequency components of the fundamental frequency and the frequency three times as high as the fundamental frequency. Accordingly, the rocking member 14 having a mirror surface is driven with a triangular wave and deflects light at a deflection angle that varies with less variation in angular speed compared to the case in which the rocking member 14 is driven with a sine wave. Oscillation of the rocking member 14 is detected by the detectors 15 and 32, and the control circuit 30 generates a drive signal necessary for obtaining a triangular wave. The drive unit 23 and the drive circuit 50 are used for driving the micromirror. Thus, when light is deflected and scanned, the angular speed is substantially constant in a region larger than that in the case in which the displacement angle varies as a sine wave. Therefore, a larger area can be used within the entire deflecting/scanning area.
The other components shown in FIG. 20 are structured as follows. That is, a displacement detection signal from the detector 32 is supplied to a band-pass filter circuit 36 via a signal line 34. The band-pass filter circuit 36 supplies only a frequency component of a first-order natural frequency oscillation mode (fundamental resonance frequency component) in the detection signal to a first signal line 38 and a second signal line 40. The signal fed via the first signal line 38 is input to a multiplier 42, where the signal is converted into a signal having a frequency three times as large as the fundamental resonance frequency. The multiplier 42 includes a phase adjustment input 54 and an amplitude adjustment input 55. These two inputs are used to adjust the phase and maximum amplitude of an output signal from the multiplier 42 so that the displacement of the rocking member 14 detected by the detector 32 varies in a substantially triangular wave in the deflecting/scanning process.
The signal from the multiplier 42 is input to an adder 46. The adder 46 adds the signal from the multiplier 42 and a frequency signal in the first-order natural frequency oscillation mode obtained through the second signal line 40 and an automatic gain control circuit 60. As a result, a drive signal for the resonant optical deflector 12 is generated. The drive signal is transmitted to the drive circuit 50 via a signal line 48, and the drive unit 23 is driven by a composite waveform of a frequency signal of the first-order natural frequency oscillation mode and a frequency signal with a frequency three times as high as the fundamental resonance frequency.
The automatic gain control circuit 60 includes a peak detection circuit 58, a difference amplifier circuit 61, a preset amplitude 63, an amplifier 62, and a gain control circuit 64. The second signal line 40 is divided into two signal lines 40a and 40b. A signal supplied via the signal line 40a is used by the difference amplifier circuit 61 to detect a difference between a maximum amplitude detected by the peak detection circuit 58 and a value of the preset amplitude 63 that is set in advance. A difference signal representing the thus-obtained difference is transmitted to the amplifier 62 that controls the gain control circuit 64. The gain control circuit 64 is controlled such that a signal having the same amplitude as the preset amplitude 63 can be obtained from the signal supplied via the signal line 40b. 
In the structure described in U.S. Pat. No. 4,859,846, the signal from the detectors 15 and 32 is divided into two frequency components using the band-pass filter circuit 36. Therefore, the circuit structure is complex and it is difficult to achieve high-accuracy control.
This problem can be solved by a method described in Japanese Patent Application No. 2006-035491 in which a oscillation system having a plurality of resonance frequencies is controlled so as to perform a desired motion. By using this method to control oscillation of a mirror, an image bearing member can be scanned with a laser beam at a constant speed.